1. Field of the Invention
The present invention relates to an optical apparatus using an optical system such as an optical element, a microscope, a lithographic optical system or an optical system of an optical disk.
2. Description of the Related Art
In the prior art, in these optical systems, in order to increase resolution, it has been proposed that an object-side numerical aperture (NA) is increased by a method such as a water-immersion method, oil-immersion method or solid-immersion method (see non-patent document 1 listed below). On the other hand, there are the following documents (non-patent documents 2 and 3, and patent documents 1 and 2) relating to materials (e.g. photonic crystal) that exhibit refractive characteristics different from those of an ordinary glass lens and the like.    (1) Non-patent document 1: “Structure and Application of Optical System”, pp. 73-77, pp. 166-170, Optronics Co., Ltd., Published on Nov. 19, 2003,    (2) Non-patent document 2: J. B. Pendry Phys. Rev. Lett., Vol. 85, 18 (2000) 3966-3969,    (3) Non-patent document 3: M. Notomi, Phy. Rev. B. Vol. 62 (2000) 10696,    (4) Non-patent document 4: L. Liu and S. He Optics Express Vol. 12, No. 20 4835-4840 (2004),    (5) Non-patent document 5: V. G. Veselago, Sov. Phys. Usp. Vol. 10, 509-514 (1968),    (6) Patent document 1: US 2003/0227415 A1, and    (7) Patent document 2: US 2002/0175693 A1.
In the prior-art methods, however, a sample surface, an optical disk surface or a wafer surface contacts a lens, water, oil or a photomask, or only a small distance of about 30 nm is provided therebetween. Thus, the working distance (WD) is too short, leading to a practical problem such as damage to a sample, a lens, etc. In order to solve this problem, an optical system using a medium with a negative refractive index was proposed in non-patent document 4.